The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. The EPD was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes is usually transparent. A suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment or the color of the solvent can be seen according to the polarity of the voltage difference.
There are several different types of EPDs. In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol. ED 26, No. 8, pp. 1148–1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movements of particles such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. No. 5,961,804 and U.S. Pat. No. 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a suspension of charged pigment particles that visually contrast with the dielectric solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors.
An improved EPD technology was disclosed in co-pending applications, U.S. patent Ser. No. 09/518,488, filed on Mar. 3, 2000, U.S. patent Ser. No. 09/759,212, filed on Jan. 11, 2001, U.S. patent Ser. No. 09/606,654, filed on Jun. 28, 2000 and U.S. patent Ser. No. 09/784,972, filed on Feb. 15, 2001, all of which are incorporated herein by reference. The EPD comprises closed cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent.
FIG. 1 illustrates a typical EPD cell 100 comprising a quantity of electrophoretic dispersion 102, the dispersion comprising a plurality of charged pigment particles 104 dispersed in a colored dielectric solvent 106. The dispersion 102 is contained by a top layer of insulating material 108 and a bottom layer of insulating material 110. In one embodiment, the insulating material may comprise a non-conductive polymer. In the cells described in the above-incorporated co-pending patent applications, the insulating layer may comprise a sealing and/or adhesive layer, or the microcup structure. The dispersion and associated insulating materials are positioned between an upper electrode 112 and a lower electrode 114.
An EPD may be driven by a passive matrix system. For a typical passive matrix system, there are column electrodes on the top side (viewing surface) of the display and row electrodes on the bottom side of the cells (or vice versa). The row electrodes and the column electrodes are perpendicular to each other.
Cross bias is a well-known problem for a passive matrix display. The voltage applied to a column electrode not only provides the driving bias for the cell on the scanning row, but it also affects the bias across the non-scanning cells on the same column. This undesired bias may force the particles of a non-scanning cell to migrate to the opposite electrode. This undesired particle migration causes visible optical density change and reduces the contrast ratio of the display.
Conventional EPD devices, such as those described in U.S. patent application Ser. No. 60/417,762 filed Oct. 10, 2002, which is incorporated herein by reference, are sensitive to environments where temperature ranges may be extreme such as an outdoor environment. When an EPD is used in an outdoor environment, it may experience temperature extremes rising to more than 80° C. or less than −20° C. When the environmental temperature exceeds 60° C. or falls below 0° C., the performance of a conventional EPD can degrade quickly. Although conventional EPDs may work well in controlled, moderate indoor environments, the outdoor temperature extremes can affect the threshold effect exhibited by EPD cells such as those described in the '762 application.
Heating devices are used in display systems to control temperatures in extreme environments. A heating device for a flat panel display should be thin, compact, and light in weight. Typically micro-wire and thin film heating devices have been used for flat panel displays. Micro-wire heaters exhibit satisfactory transparency and can be applied to the viewing side of a display. However, the costs for micro-wire heaters are generally high. The costs for thin film heaters are also high and such heaters often filter desired light from the display, thus degrading displayed images.
Thus, there is a need for an EPD that can adjust for environmental temperature extremes such as those found in an outdoor setting. There is also a need for an EPD that under extreme temperature conditions can maintain satisfactory EPD performance.